In recent years, an aspherical surface which is not symmetric with respect to the optical axis, in other words, a “free curve surface” has started to be used. Unlike an optical system that is rotationally symmetric, a free curve surface has flexibility in layout and flexibility in correcting aberrations, which makes it possible to implement a compact and high performance optical system. A prism having a free curve surface (free curve surface prism) in particular can now implement a highly precise shape due to the advancement of materials and molding technology using injected molded glass and resin, and has extremely high potential to produce a compact, high-spec and high performance optical system. However chromatic aberration is often generated in the optical system because of wavelength dispersibility of the prism material, and this causes diminished image quality. This tendency is especially conspicuous if the free curve surface prism is used in a wide wavelength range up to infrared.
Conventionally an optical system, where chromatic aberration that remains in a decentered prism having a free curve surface, is corrected by arranging a diffractive optical element (DOE) between the decentered prism having the free curve surface and an entrance pupil, is known (e.g. see Patent Document 1).